1. Field of Invention
This invention relates to flash memories and more particularly to flash memory cells created from micro vacuum tube technology.
2. Description of Related Art
A micro vacuum tube is a cold cathode field emission device in which electrons are emitted into a vacuum at room temperature under a sufficiently high electric field. The electric field does not require a high voltage to produce emission providing that the emitting surface has a sufficiently small radius of curvature. Electrons are emitted by the cold cathode past a selector gate and collected at an anode. The anode can be a floating gate of a flash memory cell. One of the advantages of the micro vacuum tube is the small area required on the surface of a semiconductor substrate. The micro vacuum tube devices can be manufactured on the surface of a semiconductor substrate using integrated circuit techniques and finally sealing the micro vacuum tube with a layer of metalization under a vacuum.
In U.S. Pat. No. 5,731,597 (Lee et al.) a field emitter array (FEA) is incorporated with MOSFET""s using common processing steps. In U.S. Pat. No. 5,651,713 (Lee et al.) describes a method for manufacture of a low voltage FEA array with minute gate holes on a semiconductor substrate. In U.S. Pat. No. 5,231,606 (Gray) is disclosed a FEA array having two or more collector electrodes, an extractor electrode, at least one deflector electrode and at least one electron field emitter.
In nonvolatile memories such as flash memories the durability of the oxide in the program and erase path is key to the longevity of the flash memory. A major issue with the development of flash memories is lessening the program and erase damage; however, it is inevitable that the oxide quality will decay and eventually end the useful life of a flash memory cell. A micro vacuum tube technology forming an FEA provides a means by which the classical degradation of an oxide does not exist because hot carriers are not used as a means to charge a floating gate. Instead a flow of electrons from a cold cathode is used to providing the charge for the floating gate.
In this invention a flash memory cell is constructed from a micro vacuum tube located over a floating gate. A sequence of oxide, polysilicon and silicon nitride is built up over the floating gate. A center hole is formed in the silicon nitride over the location of the floating gate and sidewall spacers are added to the walls of the center hole to make the diameter of the hole larger at the top and narrower at the bottom near the floating gate. The center hole is etched through to the floating gate and is partially filled with a sacrificial material. Because the center hole diameter is not uniform from top to bottom, the sacrificial material forms a depression at the center of the hole that is used to form the shape of the cathode tip of the micro vacuum tube. Support holes are formed in the sacrificial material around the peripheral of the center hole, and a conductive material such as polysilicon is formed over the sacrificial material including the depression at the center hole. The sacrificial material is etched away leaving a sharp conical shaped cathode tip formed from the deposition of the conductive material onto the depression in the sacrificial material and creating a void extending from the cathode tip to the floating gate. A high melting point metal is vacuum deposited over the conductive material forming the cathode tip sealing off the void in the hole under the cathode tip.
The shape of the cathode tip can be altered by changing the shape of the center hole before it is partially filled with the sacrificial material. For instance, an elongated hole having an oval like shape around its peripheral will produce a line like depression in the sacrificial material which when filled with the conductive material will produce an elongated cathode tip similar to a knife edge. This elongated cathode structure increases emission efficiency and provides for quicker charging of the floating gates.
The anode of the micro vacuum tube which forms the floating gate of flash memory cells can be implemented in several different ways. Each implementation entails locating a surface directly under the cathode tip that can be charged and can hold that charge for an extended period of time. In a first embodiment a floating gate is formed using polysilicon or other conductive material, and then a source and drain are formed on either side of the floating gate. The flash memory cell read using the source and drain in a standard fashion to supply current to a sense amplifier. In a second embodiment a floating gate is formed without a source or drain being formed. The cell is read by checking the re-programmability condition. If the cell is re-programmable, the floating gate must not be charged, otherwise it would be charged or programmed. In a third embodiment an area of ion implantation is made into the semiconductor substrate that accumulates charge from the tip of the micro vacuum tube. The cell is read by checking the re-programmability condition. If the cell is re-programmable, the ion implantation area must not be charged, otherwise it would be charged or programmed.